In the design of valves for controlling large flows of fluid, one of the important problems is the avoidance of excessive bulk and weight, particularly in regard to the elements that are shifted to accomplish control functions. In the case of valves that use a piston or spool as a mobile member shiftable to effect the required control function, the valve construction is relatively simple but the fluid flow capacity of the valve is small in relation to the overall size of the valve. When large fluid flows have to be accommodated, the valve size becomes excessive and in particular the piston or spool has to be of such a size and mass that its operational speed is limited.
For these reasons it is often preferred to use a poppet valve for controlling large fluid flows, because this type of valve has an intrinsically large flow capacity for a given overall size. However, poppet valves of conventional type involve a complex construction if they are required to control more than one fluid stream. Thus it is a characteristic of the type that one poppet element is required for the control of each pair of ports so that to achieve the equivalent control functions of a 5-port relay valve such as used, for instance, to control a double-acting power cylinder, a valve having four poppet elements would be required.
The complexity of a multi-port poppet valve derives from the number of poppet elements involved and the means provided for operating these in the required combinations.
The desirable flow capacity of the poppet valve design can also be achieved by the so-called sphincter valve design in which the movement of a flexible diaphragm is used to control the flow of fluid between a pair of ports. In one valve construction of this type, one port is located centrally opposite to the diaphragm so that movement of the diaphragm towards that port can be arranged to seal the port from an adjacent port. This movement of the diaphragm may be brought about by fluid pressure applied to the opposite side of the diaphragm or by mechanical means. This simple construction has however the disadvantage that because the diaphragm engages the edges of the port it is subject to wear and possible damage by such engagement.
For this reason, the more usual sphincter valve construction involves a partition between a pair of fluid flow ports, the partition presenting a ridge formation to a diaphragm that can be deflected into engagement with that formation to make a seal having line contact of a controllable width, whereby the stresses to which the diaphragm material is subjected are predictable, permitting the diaphragm to be designed appropriately to withstand those stresses for an acceptable working life.
Another problem that arises in the design of sphincter valves is that of providing the required force to deflect the diaphragm into its port-closing position, especially when the valve has to control fluid flows at relatively high pressure. Mechanical means for accomplishing the required diaphragm deflection can generate the required forces but when the diaphragm deflection is to be caused by a pilot or signal pressure fluid it is usually necessary to provide that pressure fluid with some mechanical advantage in its action upon the diaphragm. Thus, for instance, it is common to employ a large diameter piston that is acted upon by the pilot or signal pressure fluid and that serves to apply thrust to a mechanical part suitably shaped to fit the diaphragm when engaged with the seat against which the latter is to be sealed in the port-closing position.
In such a construction, some resilience in the sealing system is necessary, either in the thrust-transmitting component or in the diaphragm in order to ensure adequate sealing of the diaphragm over the whole area of contact with its seat. Even so, the relevant mating parts must be made to close dimensional tolerances if satisfactory sealing is to be ensured without the need for excessive forces. It is common to make the diaphragm of such a valve from a thick, soft elastomeric material so that its sealing area can accommodate discrepancies in the shape of the mechanical thrust-transmitting component and the seating. However, the necessary softness and thickness of the diaphragm material makes the diaphragm prone to permanent deformation and fatigue failures.